feat: install goal gate hook

[webdevtodayjason]

Summary

• install a real .claude/hooks/goal-gate.sh Stop-hook script instead of only embedding the example in settings.example.json
• update install.sh, README, Goal Protocol docs, changelog, and validation checks for the hook
• add an active GitHub Actions workflow under .github/workflows/validate.yml

Test plan

• bash -n install.sh
• bash -n .claude/hooks/goal-gate.sh
• parse .claude/settings.example.json as JSON
• parse validate.yml and .github/workflows/validate.yml as YAML
• smoke install full and minimal modes into /tmp targets
• verify installed hook exists and is executable in both modes
• exercise goal-gate.sh for missing, signed_off=true, and signed_off=false proofs
• git diff --check